010-158-42167 Ground Investigation Report

7/30/2019 010-158-42167 Ground Investigation Report

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ALDER KING

LAND AT NEWTOWN ROADHIGHBRIDGESOMERSET

GROUND INVESTIGATION REPORT

November 2006

Prepared for:Prepared by:Alder KingParsons Brinckerhoff LtdProperty Consultants29 Cathedral RoadPembroke HouseCardiff15 Pembroke RoadCF11 9HABristolBS8 3BA


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REPORT TITLE : Land at Newtown Road,Highbridge, Somerset - GroundInvestigation Report

REPORT STATUS : Final Issue

JOB NO. : FSE96762A

DATE : November 2006

PREPARED BY : .......................................(D Laver)

REVIEWED BY : ....................................(G Jones)

REVIEW CAT : B

APPROVED BY : ....................................(J Colcombe)


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CONTENTS

Page

EXECUTIVE SUMMARY 1

SECTION 1

INTRODUCTION 3

1.1 Terms of Reference 4

1.2 Background

1.3 Scope of Works 4

1.4 Report Structure 5

SECTION 2

SITE LOCATION AND DESCRIPTION 6

2.1 Site Location 7

2.2 Site Description 7

SECTION 3

SITE HISTORY REVIEW 8

3.1 Historical Site Development and Land Use 9

SECTION 4

ENVIRONMENTAL SETTING 10

4.1 Introduction 11

4.2 Geology 11

4.3 Hydrogeology 12

4.4 Hydrology 124.5 Licenses, Authorisations and Consents 12

SECTION 5

SITE INVESTIGATION WORKS 13

5.1 Introduction 14

5.2 Site Work Programme 14

5.3 Ground Conditions 15

5.4 Groundwater 16


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5.5 Soil and Groundwater Chemical Testing 16

5.6 Laboratory Geotechnical Test Results 17

SECTION 6

CONCEPUTAL MODEL 18

6.1 Introduction 19

6.2 CSM Components 19

6.3 Contamination Sources 20

6.4 Receptors 20

6.5 Pathways 20

6.6 Pollutant Linkages 21

SECTION 7

HUMAN HEALTH AND GROUNDWATER RISK ASSESSMENT 22

7.1 Context and Objectives 23

7.2 Human Health Risk Assessment - Assessment Criteria 23

7.3 Human Health Soils Risk Screening 25

7.4 Soil Gas 25

7.5 Controlled Waters Screening 267.6 Previous Risk Assessment Comparisons 27

7.7 Risk Assessment Conclusions 27

SECTION 8

GEOTECHNICAL APPRAISAL 29

8.1 Ground Condition Summary 30

8.2 Foundation Options 30

8.3 Foundation Concrete 31

SECTION 9

CONCLUSIONS AND RECOMMENDATIONS 32

9.1 Current and Future Liabilities 33

9.2 Recommended Mitigation Measures - Future Use 33

9.3 Recommended Mitigation Measures - Construction Period 34

9.4 Mitigation Costs 34

9.5 Geotechnical Issues 35


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TABLES

Table No. 1 - Soil Chemistry Summary TableTable No. 2 - Groundwater Chemistry Summary Table

Table No. 3 - Ground Gas Summary Table

LIST OF FIGURES

Figure No. 1 - Site Location Plan (1: 200,000 Scale)

Figure No. 2 - Exploratory Hole Location Plan (1 : 1,250 Scale)

Figure No. 3 - Soil and Groundwater Exceedance Summary Plan (1 : 1,250 Scale)

Figure No. 4A - Conceptual Site Model (CSM) Existing

Figure No. 4B - Conceptual Site Model (CSM) Future Use

APPENDICES

APPENDIX A - EXPLORATORY HOLE LOGS

APPENDIX B - CHEMICAL TEST RESULTS

APPENDIX C - GEOTECHNICAL TEST RESULTS

APPENDIX D - CLEA UK DATASHEET SUMMARIES


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EXECUTIVE SUMMARY


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LAND AT NEWTOWN ROADEXECUTIVE SUMMARY HIGHBRIDGE, SOMERSET

EXECUTIVE SUMMARY

Alder King commissioned Parsons Brinckerhoff Ltd (PB) to undertake a combined geo-environmental and geotechnicalinvestigation to form part of a technical sales package for prospective developers of the study site and to support thepreparation and submission of an outline planning application for residential development at the site. The site resides alongNewtown Road, in Highbridge Somerset.

The site investigation works were carried out between the 18 and 24 October 2006 and comprised the excavation of 8 trial pitsand construction of 2 combined groundwater and gas boreholes.

Environmental Risk - Human Health

Risks to human health are low for the current land use. This risk rating should remain low for a residential end use scenarioprovided a pragmatic approach to development is adopted. The human health mitigation approach (clean cover)recommended in 2001 remains valid in light of the current 2006 data set. It is recommended that the existing developmentplatform is raised by a minimum 600 mm above existing.

The following general sequence for ground preparation is recommended.

Existing building demolition followed by concrete segregation and crushing.

Targeted ground investigation below foundation slabs, particularly along the western site boundary of the site.

Targeted excavation and removal of contamination hotspots identified during all investigations (a source removalapproach).

Provision of a nominal thickness of clean cover layer between any remaining Made Ground soils and future siteusers.

Environmental Risk - Controlled Waters

Whilst human health risks may be concluded to be low, the prevailing groundwater quality below the site suggests aprecautionary approach to dealing with residual contaminant sources may be required. Limited mitigation steps (primarilysource removal in certain areas) are warranted and should be encouraged in order to address a possible long term impact onsurface water quality adjacent to the site.

Environmental Mitigation Costs

Budget off-site disposal costs (1000 m3

of site soils), could range between 45,000 and 100,000 depending on wasteclassification. Early engagement with the regulatory authorities is recommended with a view to minimise the need for off-sitedisposal where possible. The latter will however require a Waste Management Licence exemption provided the soil is suitablefor reuse. Waste Acceptable Criteria (WAC) testing should be undertaken as a matter of course to provide better cost certainty.

Approximately 15% of the materials required to raise the development platform should be available on site (site won crushedconcrete). The remainder (a conservative estimate of 13,000 m

3to provide the minimum desired 600 mm cover), will need to

be sourced through an off-site supplier. Approximate unit costs to import, place and compact a suitable granular fill to anengineering specification would be in the region of 18 - 24/m

3.

Geotechnical Considerations

In general foundation solutions for the development should be relatively straightforward to implement. Foundation options arelikely to include the use of either ground bearing rafts or piles deriving their load capacity from the underlying marine sands.The ultimate selection of foundation solution will depend on the type of construction envisaged and general development layout(both of which are yet to be confirmed). Conventional two storey development may be able to proceed using shallowfoundation types such as ground bearing rafts founded on compacted fill (with perhaps some preparatory groundimprovement), whereas higher rise development is likely to require a deeper, more robust solution such as a piled foundation.

Foundation Concrete

The Aggressive Chemical Environment for Concrete (ACEC) classification for this site is AC-1, in accordance with BRE SpecialDigest 1, Concrete in Aggressive Ground (2001).

FSE96762A Report by Parsons BrinckerhoffNovember 2006 Page 2 for Alder King


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SECTION 1

INTRODUCTION


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SECTION 1 LAND AT NEWTOWN ROADINTRODUCTION HIGHBRIDGE, SOMERSET

1 INTRODUCTION

1.1 Terms of Reference

1.1.1 Alder King commissioned Parsons Brinckerhoff Limited (PB) to undertake a combinedGeoenvironmental and Geotechnical Investigation at Newtown Road, Highbridge inSomerset (the Study Site).

1.1.2 The investigation has been undertaken in accordance PBs Proposal (ReferenceGJ/BD2006-Highbridge 060922) dated 22 September 2006.

1.2 Background

1.2.1 Alder King wishes to prepare a technical sales package for the Study Site, to informprospective purchasers and the Regulatory Authorities of ground conditions and thesteps that may be required to realise a residential development. Information on theabove is also being sought in support of an outline planning application for the site.

1.2.2 A preliminary contamination assessment was undertaken for the same site area in2001. Environmental risk assessment methodology has advanced significantly sincethis time however and an up-dated more robust assessment has been considerednecessary to bring site understanding in line with current best practice guidance. Inaddition and further to the above, the 2001 scope of work did not consider potentialgeotechnical constraints on future development. Geotechnical information relevant todevelopment is now considered an important part of the ultimate technical salespackage.

1.2.3 The preliminary, indicative development layout for the site shows residential housing,of various construction type, extending across the majority of the available area

(approximately 80 - 100 No. dwellings).

1.3 Scope of the Works

1.3.1 In line with the above, the focus of this study has therefore been to conduct anappropriate ground investigation capable of providing greater detail on (1) groundcontamination issues and (2) to provide an indication of likely foundation requirementsfor an assumed residential development scenario. An intrusive investigation of thesite has therefore been designed and carried out to provide information on potentialcontamination liabilities and indicative ground conditions with respect to suchproposed development. It should be noted that the potential for Asbestos ContainingMaterials (ACMs) in building fabric has not been assessed. Further, accesslimitations (above ground structures), precluded any assessment of soil quality along

the western site boundary.

1.3.2 This report presents information gathered during an environmental and geotechnicalintrusive investigation carried out across the proposed development footprint. A standalone desk study review was not commissioned as part of this commission; howeverthe desk based information available at the time of the 2001 report has been reviewedonce more and incorporated into this phase of work.

1.3.3 The scope of works undertaken as part of this investigation comprised the following:

A review of the findings of previous study across the site and the data/mappingsearches that were conducted.



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SECTION 1 LAND AT NEWTOWN ROADINTRODUCTION HIGHBRIDGE, SOMERSET

A review of the geology, hydrology and hydrogeology of the site.

An intrusive investigation across accessible areas of the site which has includedsoil logging, sampling of site soils and in-situ geotechnical testing of the ground.

Scheduling of analytical testing for a range of organic and inorganiccontaminants, and geotechnical laboratory testing on a limited range of soilsamples recovered from investigation locations.

A tiered environmental risk assessment of the site with respect to the impact onfuture users of the site and the general environment and a geotechnical appraisalof ground conditions, presenting likely foundation options for development.

1.4 Report Structure

1.4.1 A description of the site, its history and environs follows this introduction as SectionNos. 2 to 4. Section 5 presents the findings of the site investigation followed bydetails on the Conceptual Site Model, Contamination Assessment and Geotechnical

Appraisal presented in Sections 6 to 8. The conclusions and recommendations arepresented under Section 9.



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SECTION 2

SITE LOCATION AND DESCRIPTION


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SECTION 2 LAND AT NEWTOWN ROADSITE LOCATION AND DESCRIPTION HIGHBRIDGE, SOMERSET

2 SITE LOCATION AND DESCRIPTION

2.1 Site Location

2.1.1 The site is located approximately 10 km north of Bridgwater and 2.5 km south east ofBurnham on Sea. The National Grid Reference of the site is 331550E 147390N andthe site is at an elevation of approximately 6 m AOD (metres Above OrdnanceDatum). The confines of the site are accessed directly from Newton Road at itseastern limit. A site location plan is presented as Figure No. 1.

2.2 Site Description

2.2.1 The proposed development site (the Study Site) is approximately triangular in shape,tapering to a point at its eastern boundary. The site surface is reasonably level andthe boundary encloses an area of approximately 2 hectares. The site is currentlyoccupied by engineering premises and miscellaneous storage units, some tight upagainst the western boundary. Hard surface coverage around existing structuresconsists of either tarmac or concrete and occupies as much as 90% of the total sitearea. The remaining 10% of the site area comprises rough or soft ground cover.

2.2.2 Land use bordering the site is typified by residential housing to the north, east andsouth. In addition to residential land use, Wessex Water operate a sewage works tothe west of the site boundary with sewage outfall pipe-work shown to run within andalong the southern boundary in an easterly direction.

2.2.3 Rough ground extends beyond the southern boundary of the site (under which theformer channel of the Old River Brue is understood to lie). A small boatyard andassociated storage units are situated further south, close to the New Clyce Bridge andlock system.



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SECTION 3

SITE HISTORY REVIEW


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SECTION 3 LAND AT NEWTOWN ROADSITE HISTORY REVIEW HIGHBRIDGE, SOMERSET

3 SITE HISTORY REVIEW

3.1 Historical Site Development and Land Use

3.1.1 The site history and associated changes of land use through time has been reviewedpreviously using environmental information contained within an EnviroCheck report(Parsons Brinckerhoff - 2001). The information available at the time of the 2001 studypresents reasonably comprehensive listings and historical mapping detail between1887 and the present day. A summary review of this information and the key featuresidentified is presented and discussed as follows:

Site and Immediate Surrounding Area

3.1.2 Between 1887 and 1986 the key developments are the general flux of industrialoccupation either within or the immediately adjacent to the site boundary. Severalseparate periods of development are apparent during this time period comprising inthe main, variation in the layout of saw mills and their connection with adjacentlanding wharfs along the old River Brue.

3.1.3 The River Brue itself undergoes significant re-alignment and straightening during thesame period with a river meander (Circa 1978), suspected to have been infilled withinert construction waste. This former meander (referred to as the Old River Brue),once flowed east-west within 50 m of the southern boundary of the Study Site.

3.1.4 On the Study Site itself, several contiguous structures are shown from as early as1904 remaining up until at least 1978. These are believed to have been associatedwith the saw mill operations noted above and appear to be linked directly with variousrailway lines and sidings.

3.1.5 A patent fuel works is shown west of the study site boundary between 1930 and 1962.During this time, this facility is shown situated across the area now occupied by aSewage Works. The sewage works is noted for the first time in 1978.

3.1.6 The earliest development to the north of the Study Site is the apparently rapidexpansion of the Britannia Brickworks and clay pits close to the northern boundaryfrom as early as 1887. The brickworks appear to be taken over by the saw milloperations, which are continually expanding from the south, by approximately 1931.

3.1.7 The rate of residential development around the site starts to increase by around 1962,constraining the saw mill operations to within the study site boundary. It is not until1992 that the saw mill operations themselves are no longer shown.

3.1.8 In general, the tabulated site history presented as part of the 2001 study is anaccurate record of the key developments across and within influence of the StudySite.



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SECTION 4

ENVIRONMENTAL SETTING


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SECTION 4 LAND AT NEWTOWN ROADENVIRONMENTAL SETTING HIGHBRIDGE, SOMERSET

4 ENVIRONMENTAL SETTING

4.1 Introduction

4.1.1 The geological and hydrogeological site setting has been reviewed as part of previousstudies (PB 2001), but for completeness is presented here again. Reference hasbeen made to the following data sources:

The 1:50,000 British Geological Survey (BGS) Sheet 279 and Parts of 263 and295 Weston Super Mare (Solid and Drift Edition);

The Environment Agency (EA) Policy and Practice for the Protection ofGroundwater, Groundwater Vulnerability Sheet 42 Somerset Coast.

4.2 Geology

4.2.1 The geology and hydrogeology of the study area is summarised in the following tableand expanded below:

Table No. 4.2 - Geology and Hydrogeology

Formation Description AquiferClassification

Flow Mechanism(if applicable)

Made Ground Undifferentiated NA -

Estuarine clay andBurtle Beds

Soft to firm grey silt/clay (organic richlenses), overlying shelly sands and

gravels.

Non aquifer -

Lower Lias Undifferentiated Mudstones andsiltstones with some limestone.

Non Aquifer(negligiblypermeable).

Fracture flow

Solid Geology

4.2.2 The 1 : 50,000 British Geological Survey (BGS) Sheet 279 and Parts of 263 and 295Weston Super Mare (Solid and Drift Edition), indicates that the natural solid geologyunderlying the majority of the site at depth is Jurassic Lower Lias deposits comprisingundifferentiated mudstones with some limestone, overlain in part and in its upper

reaches, by siltstones.

Drift Deposits/Made Ground

4.2.3 Estuarine or marine sands and silts/clays overlie the solid geology. Cohesivedeposits are generally soft grading firm and grey in colour. The thickness of thesedeposits is considered likely to be in the region of 10 m. The estuarine deposits arelikely to be overlain by a variable thickness of Made Ground across much of the StudySite (on account of its industrial past). The intrusive work undertaken in 2001encountered up to 1.5 m Made Ground soils of variable composition (mainly ashy fillwith varying amounts of builders rubble).



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SECTION 4 LAND AT NEWTOWN ROADENVIRONMENTAL SETTING HIGHBRIDGE, SOMERSET

4.3 Hydrogeology

4.3.1 Information on the hydrogeology of the region has been obtained from theEnvironment Agency (EA) Policy and Practice for the Protection of Groundwater,

Groundwater Vulnerability Sheet 42 Somerset Coast.

4.3.2 The Lower Lias - Jurassic mudstones and siltstone that underlie the site is classifiedby the Environment Agency as a Non Aquifer (although the unit is considered to benegligibly permeable). The presence of the layer of (assumed) low permeabilityEstuarine Alluvium overlying this unit will minimise the potential for vertical leaching ofany Made Ground contamination to this unit. It is possible that the estuarine claymay, in part be discontinuous or of variable thickness depending on the presence ofpeat layers of over thickening of the shelly Burtle Bed sands. If and where this is thecase, there will be a greater hydraulic potential between the Made Ground unit andthe underlying bedrock.

4.3.3 The site is within 50 m (north) of the former Old River Brue meander. It is suggested(although not confirmed through intrusive investigation), that this former section ofwater course has been filled with a builders type rubble fill. It is feasible that thisfeature may more readily transmit groundwater than the surrounding natural cohesivedeposits and in parts is likely therefore to afford some degree of hydraulic connectivitybetween adjacent Made Ground deposits and the tidal reaches of the River Brue.

4.3.4 Information supplied by the Landmark Information Group indicates that there are noEA licensed groundwater abstractions within 500 m of the site.

4.4 Hydrology

4.4.1 The nearest watercourse shown on current plans is the tide locked/tide controlled

lower reaches of the River Brue. This section of river is located approximately 100 msouth of the southern site boundary and when permitted, flows west into the tidalreaches of the Severn Estuary. The mouth of the estuary and the receiving waters ofthe River Brue are designated as a Site of Special Scientific Interest (SSSI) andRamsar designated under European legislation. Water quality however is reported tobe moderate to poor.

4.5 Licences, Authorisations and Consents

4.5.1 Information supplied by the EA indicates that there are no surface water abstractionpoints within 500 m of the site, but there are 2 No. surface water discharge consentswithin 500 m of the Study Site, both operated by Wessex Water and relating tosewage discharge within the Brue catchment area.



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SECTION 5

SITE INVESTIGATION WORKS


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SECTION 5 LAND AT NEWTOWN ROADSITE INVESTIGATION WORKS HIGHBRIDGE, SOMERSET

5 SITE INVESTIGATION WORKS

5.1 Introduction

5.1.1 The objectives of the investigation were to interrogate the initial Conceptual SiteModel (see section 6) and provide geotechnical information. This involved intrusiveinvestigation to provide the following:

To record the soil strata encountered.

To make observations of groundwater occurrence and behaviour.

Investigate where possible, environmental liabilities associated with groundconditions at the site.

To determine the geotechnical properties of ground conditions at the site; and,

To provide an environmental risk assessment and geotechnical appraisal of the

ground conditions at the site with respect to future below ground excavations andfoundation construction.

5.2 Site Work Programme

5.2.1 The site investigation was carried out on the 18 and 24 October 2006 and comprised:

Excavation of 8 trial pits to depths of between 2.4 m and 4.3 m below groundlevel; and,

Excavation of 2 Cable Percussion boreholes holes to depths of between 10 mand 13 m below ground level (bgl).

5.2.2 The locations of all exploratory holes are shown on Figure No. 2. Each location waspositioned in accordance with EA Guidance (CLR4 and Secondary Model proceduresfor the Development of Appropriate Soil Sampling Strategies for land Contamination P5-066/PR), and to complement the layout of positions completed in 2001.

5.2.3 The ground investigation works were carried out in accordance with BS 5930 : 1999Code of Practice for Site Investigations and Specification for Ground Investigation(Thomas Telford 1993), and in accordance with BS 10175: 2001 (Investigation ofPotentially Contaminated Sites). All works were completed without incident.

5.2.4 Trial Pit excavations were used for exploratory purposes and to enable representativesoil samples to be recovered for analytical testing. The boreholes were used tocollect soil samples for laboratory geotechnical testing, to undertake insitu

geotechnical testing at depth (Standard Penetration testing) and to provide samplingpoints for groundwater sampling and gas monitoring.

5.2.5 Records of all exploratory holes, detailing the depth and descriptions of the strataencountered, depths of samples taken and the results of insitu geotechnical testingare presented in Appendix B.



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5.3 Ground Conditions

5.3.1 A summary of ground conditions is presented in Table No. 5.3 below:

Table No. 5.3 - Ground Condition Summary

Maximum Proven Thickness (metres)Location

Made Ground Alluvium Peat Marine Sands

BHA 2006 0.4 7.0 1.9 3.7*

BHB 2006 0.3 6.0 - 3.7*

TP01 2006 0.65 3.25* - -

TP02 - 2006 0.55 2.05* - -

TP03 2006 0.8 2.8* 0.1 -

TP04 2006 1.2 3.1* - -

TP05 2006 1.1 3.1* - -

TP06 2006 1.1 2.9* - -

TP07 2006 0.6 1.8* - -

TP08 - 2006 0.7 3.3* - -

BH01 - 2001 1.5 1.6* - -

BH02 2001 0.7 3.3* - -

BH03 2001 0.56 2.44* - -

BH04 2001 1.0 2.0* - -

BH05 2001 1.5 2.5* - -

BH06 2001 0.8 2.2* - -

BH07 - 2001 0.55 2.45* - -

BH08 2001 0.6 1.4* - -

BH09 2001 0.75* - - -

Unit thickness range where proved(metres)

0.3 1.5 6.0 7.0 1.9 -

SPT (N) Range - 1 - 14 - 16 32

SPT (N) Averages excludingrefusals

- 6 - 24

*Full thickness not proven

Made Ground

5.3.2 Tarmac was typically encountered from ground level at each exploratory hole locationto depths of approximately 0.1 m. Below hard cover layers, Made Ground was

encountered in all the exploratory holes. This was the case for the investigationsundertaken in 2001 as well as those completed in 2006. The full depth of MadeGround was proven at all locations, with thickness ranges between 0.3m and 1.5m.Typically very shallow across the site (


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Natural Strata

5.3.4 The natural strata below the site, proven through intrusive investigation in 2001 and2006 comprises estuarine alluvium and at depth, marine sands. The bedrock was not

proven during either the 2001 or 2006 work. The estuarine alluvium has been foundto consist of a grey, firm to stiff occasionally soft to firm clay. The organic content ofthe clays was found to be reasonably high and at depth (BH-A), a 1.9 m thick peatband was recorded.

5.3.5 Underlying the peat in BH-A, and below the estuarine clays in BH-B, fine silty marinesands, were proven to a maximum depth of 13 m bgl.

5.4 Groundwater

5.4.1 Shallow excavations in the upper reaches of the estuarine clay were generally dry,however minor water seepages were noted on occasion either along the underside ofthe granular Made Ground, or along open discontinuities in the clay. This was foundto be the case during the 2001 investigation.

5.4.2 Similarly, during the drilling of the two deep boreholes (2006 investigation), perchedwater strikes were rare and minor, quickly sealed by the advancing borehole casing.The main water strike was encountered at the interface between the estuarine clayand the marine sands at depths of 9.3 m and 6.5 m, in BH-A and BH-B respectively.

5.4.3 Water levels in both boreholes were monitored for a period of twenty minutesfollowing each strike, with equilibrium rest water levels stabilising at approximately6.3 m bgl.

5.5 Soil and Groundwater Chemical Testing

5.5.1 A total of 33 No. soil samples (selected from both natural and Made Groundmaterials), were recovered from the trial pits and boreholes for selective follow uplaboratory analysis. Groundwater samples were recovered following the installation ofstandpipes at each borehole location. All samples were placed in cool boxes anddespatched to the nominated accredited laboratories within 24 hours of sampling.

5.5.2 In accordance with the CSM and previous site investigation, analytical testingundertaken on selected soil samples and on each groundwater sample includedpriority determinands under the CLEA guidance, as well as pH, nickel, copper,cyanide, sulphate, mercury, selenium, zinc, speciated PAH, TPH and volatile organiccompounds. Asbestos soil screening was also carried out as a matter of course.Analysis was undertaken by a UKAS laboratory which implements MCERTS

procedures.

5.5.3 The analytical test results are presented in their entirety under Appendix B anddiscussed further in Section 7.

5.5.4 The sampling and analysis undertaken during the recent 2006 investigationsupplements the data gathered in 2001 by providing a larger dataset on which tocomment and allowing for a greater level of chemical speciation for the purpose ofcontemporary risk assessment.



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5.6 Laboratory Geotechnical Test Results

5.6.1 Laboratory geotechnical testing was undertaken on natural soils recovered from theboreholes to corroborate geological descriptions. Particle Size Distributions (PSD)

and Atterberg Limit testing were performed to assess soil material classification andwater soluble sulphate contents were determined to understand ground aggressivitywith respect to foundation concrete design. The results of the geotechnical laboratorytesting are presented in Appendix C.



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SECTION 6

CONCEPTUAL MODEL


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SECTION 6 LAND AT NEWTOWN ROADCONCEPTUAL SITE MODEL HIGHBRIDGE, SOMERSET

6 CONCEPTUAL SITE MODEL

6.1 Introduction

6.1.1 The Conceptual Site Model (CSM) is presented as Figure Nos. 4-A and 4-B (existingand proposed conditions respectively). The details and components of the CSM aresummarised below, including information on site geology, hydrology, hydrogeologyand relevant pollutant linkages.

6.2 CSM Components

Geology

6.2.1 The geology of the site is reported in more detail in Section 4. Essentially a threelayer system, the sequence may be summarised as follows:

Made Ground comprising predominantly granular, and sometimes ashy soils -occasional hydrocarbon odours. Maximum proven thickness of Made Groundunit is 1.5 m.

Estuarine Alluvial Clays, with discontinuous Peat horizons or bands. Proventhickness of Estuarine clay layer is 7 m.

Marine Sands (Burtle Beds). A maximum proven thickness of 3 m althoughexpected to extend to great depth below the site.

Hydrology

6.2.2 The surface water hydrology is dominated by the position of the Brue and Severn

Estuaries, the former located within 100 m of the southern Study Site boundary.Surface water levels within the River Brue are fixed at levels dictated by lock gates atthe New Clyce Bridge. Beyond the lock gates, further to the west, surface waterlevels are controlled by tidal water fluctuation within the main estuary.

6.2.3 It has been noted that the line of a sewer follows the southern boundary of the StudySite. As detailed in Section 4.5, there are two separate discharge consents held byWessex Water Services Ltd relating to sewage discharges to the River Brue Pill.Discharge points are recorded as being at least 400 m west of the Study Site,although their exact locations have not been explored further.

Hydrogeology

6.2.4 Very little perched groundwater has been observed/recorded in the upper superficialdeposits, either in the Made Ground or the estuarine clays. The main groundwaterbody is believed to be the underlying marine sands which exist at depths rangingbetween 6 m bgl and 9 m bgl. Rest water levels have been recorded at approximately6 m bgl, and are slightly confined by the overlying estuarine sequence. Although notconfirmed at present, it is possible that the groundwater levels measured in themarine sands will be tidally influenced by the receiving waters of the estuary.

6.2.5 The tidal range and Normal Tidal Limits depicted on historical and contemporarytopographic maps suggest that at high tide, there may be direct interaction betweentidal waters and the suspected infill materials that are said to reside within the OldRiver Brue meander. A degree of hydraulic continuity may also exist between upperMade Ground soils and the underlying aquifer.



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6.3 Contamination Sources

6.3.1 The principle contamination sources associated with the Study Site are consideredbelow. Source terms are referenced 1 and 2 and cross refer to the CSM illustrations

presented as Figure Nos. 4-A and 4-B.

Source 1: Either residual hydrocarbons (TPH and/or PAH compounds), heavymetals or ACM bound within the heterogeneous Made Ground soils.

Source 2: Soil gases associated with shallow Made Ground soils and/or naturalorganic rich estuarine clays.

6.4 Receptors

6.4.1 There are three main receptor groups for this site. These, in order of priority are asfollows and again cross refer to the CSM illustrations, Figure Nos. 4-A and 4-B:

Receptor Group 1: Future residents living at the proposed development or siteconstruction workers during the development phase;

Receptor 2: The Controlled Water environment (principally the surface waters).

6.4.2 Receptor Group 1: The residents of new development at the site are likely tocomprise a wide range of age groups from young children to the elderly. Although themajority of the development footprint is likely to comprise multi-storey typeaccommodation it is conceivable that a proportion of low rise housing with privateback garden areas could form part of the overall development plan. As such, should aviable ingestion pathway for soil contamination exist, or should there be an inhalationpathway for soil gases, long term exposure could lead to an adverse effect on thisreceptor group.

6.4.3 Receptor Group 1: During any construction phase, site construction workersrepresent a potentially significant short term receptor group to the contaminationsources identified above. Ingestion pathways would be of particular concern, howeverthere could also be inhalation vapour risks associated with any residual hydrocarboncontamination in shallow Made Ground soils.

6.4.4 Receptor Group 2: Controlled waters (in this case local surface water bodies)represent a receptor which is potentially at risk from the sources of contaminationhighlighted above. The site is in close proximity to receiving waters of the River Bruewhich are designated as a Site of Special Scientific Interest (SSSI) and Ramsardesignated under European legislation. This water body as a receptor tocontamination could be at risk from site contamination should a viable pathway exist

either (1) directly via the infilled Old River Brue channel or (2) indirectly via thegroundwater table in the marine sands below the site.

6.5 Pathways

6.5.1 With respect to both human health and controlled water risk, the main pathways tocontamination are as follows:

Pathway 1 Ingestion of hydrocarbon or heavy metal contaminated soils.

Pathway 2 Inhalation of ground gases, wind borne dusts (dry periods), orhydrocarbon vapours.



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Pathway 3 Leaching of heavy metals or hydrocarbons through the soil profile.

6.6 Pollutant Linkages

6.6.1 The viable pollutant linkages between the sources, pathways and receptors discussedabove are tabulated as follows:

Relevant Pollutant Linkages

Contaminant Source(s) Pathway(s) Receptor Group(s)

Pathway 3: Leaching and off-sitemigration of any dissolved or free-phase contaminants to perchedgroundwater, via damaged surfacewater sewers.

Receptor Group 2: SurfaceWater Bodies (River Brue).

Pathway 3: Leaching andmigration of aqueous and free-phase contaminants intounderlying Marine sands.

Receptor Group 2: SurfaceWater Bodies (River Brue).

Source 1: Polycyclicaromatic hydrocarbons(PAHs), volatile organiccompounds (VOCs) andtotal petroleumhydrocarbons (TPH)

Source 1: Heavy metals,metalloids and AsbestosContaining Materials(ACMs)

Pathway 1: Ingestion, inhalationand dermal contact withcontaminated soil.

Receptor Group 1: Current siteusers/trespassers; Futureconstruction workers; Futureresidential occupants.

Source 2: Soil gas(methane, hydrogensulphide, carbon dioxide,carbon monoxide)

Pathway 2: Inhalation of gasesmigrating laterally from infilled OldRiver Brue channel, or from theorganic rich soils below the site.

Receptor Group 1: Future siteoccupiers/residents.



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SECTION 7

HUMAN HEALTH ANDGROUNDWATER RISK ASSESSMENT


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SECTION 7 LAND AT NEWTOWN ROADHUMAN HEALTH AND GROUNDWATER HIGHBRIDGE, SOMERSETRISK ASSESSMENT

7 HUMAN HEALTH AND GROUNDWATER RISK ASSESSMENT

7.1 Context and Objectives

7.1.1 This section makes use of the site investigation findings, as described above, toevaluate further the potential pollutant linkages identified in Section 6. A combinationof qualitative and quantitative techniques has been used for human health andcontrolled waters risk assessment. Each aspect is discussed in turn below.

7.2 Human Health Risk Assessment - Assessment Criteria

7.2.1 Specific numerical assessment criteria have been used to interpret the chemicaltesting results, as described in this section. These criteria are generally set to behighly conservative and in the event that they are exceeded a further level of analysisis typically required.

Soils

7.2.2 The assessment criteria used for the screening of determinands within soils areidentified within Table No. 7.2a below. Details of input parameters are given withinthe footers to Table No. 1, presented at the rear of this report.

Table No. 7.2a: Selected Assessment Criteria Contaminants in Soils

Substance Group Determinand(s) Assessment CriteriaSelected

Organic Substances

Total Petroleum Hydrocarbons(TPH), plus VOCs

C10-C35 Aliphatic and AromaticCompounds

CLEA UK

Polycyclic aromatichydrocarbons (PAHs).

EPA 16 (PAH) CLEA UK

Inorganic Substances

Arsenic, Cadmium, Lead, Nickel,Selenium, Mercury, Chromium

CLEA (SGV)Heavy metals and metalloids.

Copper, Zinc CLEA UK

Phenols Monohydric (total) phenols CLEA UK

Asbestos Containing Materials All Asbestos Groups Presence / Absence

7.2.3 A brief discussion of the source of screening criteria is provided below.

CLEA/CLEA UK

7.2.4 In March 2002 the Contaminated Land Exposure Assessment (CLEA) model andsupporting documentation were released by the Department of Environment Foodand Rural Affairs (DEFRA) to provide a scientifically based framework for assessingchronic risks to human health posed by land contamination in the United Kingdom.CLEA has become an integral part of the risk-based approach to assessingcontaminated land in the UK.

7.2.5 In November 2005, the Environment Agency released the software CLEA UK v.1.0,which supersedes the previous CLEA software. Within CLEA UK, site-specificassessment criteria (SSAC) can be derived when used in conjunction with thepublished toxicological data released by DEFRA, which forms part of a tier 3 riskassessment. This approach is used herein.



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7.2.6 Notwithstanding the above, a Cabinet Task Force has been set up to review thecurrent Soil Guidance Value and the whole process involved in their generation. Thisis because the Task Force has released a note (CLAN Note 02/05) stating thatexceedance of the SGV is not considered as representing significant risk of

significant harm or unacceptable intake as stated in Part 2A. Therefore there iscurrently a debate as to what the government derived numbers actually mean in alegal context.

7.2.7 A further note (CLAN 4/06) has been released and no other toxicological reports orSGVs will be released until the work by the Soil Guideline Task Force is reviewed.

7.2.8 All contaminants without published SGV reports are screened against criteria derivedusing the CLEA UK model. All assumptions and input parameters (both toxicologicaland fate and transport) for these contaminants, in addition the full CLEA UK reportsummaries, are provided within Appendix D.

CLEA UK Input Criteria

7.2.9 The input criteria used for the development of Generic Assessment Criteria (GAC)within CLEA UK, and where appropriate, those required for selection of the correctscreening criteria from SGV reports, are provided within Tables No. 7.2b.

Table No. 7.2b: CLEA UK Input Criteria Residential Without Plant Uptake

Input Details Value

Land Use Residential without plant uptake

Building Type Typical House

Receptor Female

Age Class 1 6

Exposure Duration 6 yearsAveraging Time 6 years

Direct Soil IngestionOral

Direct Soil Derived Indoor Dust Ingestion

Skin Contact with Soil Derived Indoor DustDermal

Skin Contact with Soil

Inhalation of Soil Derived Indoor Dust

Inhalation of Soil Dust

Inhalation of Soil Vapours Indoors

Inhalation

Inhalation of Soil Vapours Outdoors

Soil Type Sandy

pH 8

Soil Organic Matter 5.0% (Conservative for this site setting)



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7.3 Human Health Soils Risk Screening

7.3.1 A residential land use without plant uptake for human consumption has been used to

assess potential risks to human health. In the absence of published SGV toxicologicaldata for certain parameters, a generic assessment has been made using CLEA UK.Where applicable, Site Specific Assessment Criteria have been developed to resolveany uncertainties in the level of risk to the relevant receptor groups. The assumptionsmade and provenance of data used in the model are shown in Appendix D.

7.3.2 The analytical results for soils are provided in their entirety as Appendix B. Gasmonitoring results are summarised within Table No. 3. A comparison of the testingresults with the relevant assessment criteria outlined in Table Nos. 7.2a and 7.2b ispresented in Table No. 1 with soil exceedance details illustrated on Figure No. 3. Adiscussion of the results is presented below.

Inorganic Contaminants (Metals)

7.3.3 Soil metal concentrations are generally low for all determinands. Screening againstpublished Soil Guideline Values (SGV) has only highlighted a elevated arsenicconcentrations in site soils, with levels ranging between 7 mg/kg and 29 mg/kg.Approximately 40% of the sample population were found to be at or above the20 mg/kg risk threshold used for a residential scenario, without plant uptake. TheGeneric and Site Specific Assessment Criteria (GAC and SSAC) subsequentlygenerated for arsenic (21.9 and 22.5 mg/kg respectively), which are modelled andadjusted for soil pH and organic matter, reduces the number of exceedances for thisdeterminand from three (3), to two (2). Using SSAC, arsenic concentrations exceedthe relevant threshold criteria in TP04 (4.3 m depth) and TP06 (0.4 m depth).

7.3.4 For metal determinands where SGV are not generally available (e.g. for zinc,vanadium, barium and copper), CLEA UK Generic Assessment Criteria (GAC) havebeen used. For these determinands, only barium exceeds the relevant threshold(TP08 and 0.75 m depth).

7.3.5 Other than the above, soil metal concentrations are generally well below relevantscreening criteria and are not considered to represent a significant risk to humanhealth.

Organic Contaminants (PAH and TPH Compounds)

7.3.6 For the vast majority of the organic contaminants (with the exception of toluene andethyl-benzene), GAC have been used to screen the dataset. Organic contaminant

levels are generally low and despite olfactory evidence during site work at severallocations, are well below assessment thresholds. The only exceedance noted was fornaphthalene, detected in TP02 at a depth of 0.4 m bgl. At this location naphthalenelevels of 36.7 mg/kg were found to exceed the GAC of 33.6 mg/kg (marginally abovethe modelled threshold value).

7.4 Soil Gas

7.4.1 Soil gas concentrations and flux (flow) were measured shortly after the completion ofthe 2006 investigation (26

thOctober 2006), using an infra-red gas analyser.

Measurements were taken from the two new borehole installations and from two ofthe shallow wells completed in 2001. The monitoring exercise was carried out over aperiod of rising atmospheric pressure, ranging 998 mb to 1002 mb over a period of



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approximately 5 hours. The results of the gas monitoring exercise are presented onTable No. 3.

7.4.2 2006 Monitoring Wells: Relatively high concentrations of methane (CH4) were

recorded in BH-A (38.8% steady). Oxygen concentrations were depleted at 9.9% with4 ppm Carbon Monoxide recorded and 3.2% Carbon Dioxide. Gas flow wasnegligible, fluctuating around zero (l/hr) and within the margin of error for theinstrumentation. Although gas speciation would be required as confirmation, it islikely that these concentrations are associated with a natural gas source, given thehigh organic content of the estuarine clays below the site, and the presence of a1.9 m thick band of Peat at this location. Given the response zone of this installationis below the measured water table (screened between 8.8 m bgl and 13 m bgl), it islikely that the recorded gas concentrations here are being liberated out of solutionfrom the groundwater column.

7.4.3 Conversely, methane concentrations and flow in BH-B are negligible with peak CH4concentrations at 1.1%. Carbon Dioxide concentrations remained steady at 0.7%.Low gas concentrations in this borehole could be due to the absence of significantPeat banding within BH-B.

7.4.4 2001 Monitoring Wells: Two wells installed in 2001 were found to be serviceable andtherefore included in the gas monitoring exercise (BH1 and BH8). Methane was notdetected in either borehole, although Carbon Dioxide concentrations were slightlyelevated ranging 0.4% to 0.7%.

7.4.5 On the basis of a single round of monitoring, gas concentrations overall have beenshown to be low. Gas generation rates are similarly low (based on flow rates).Anomalous, relatively high methane concentrations have been recorded in one of thedeep boreholes (BH-A). These are believed to be associated with natural gas

production within organic rich soil horizons at depth. Further gas monitoring ofshallow soils would be needed to establish gas trends and to guide design, but basedon existing data (primarily the concentrations of Carbon Dioxide), it is likely that someform of gas protection measures will be necessary immediately below or at foundationlevel.

7.5 Controlled Waters Screening

7.5.1 Groundwater chemistry below the site, together with site soil leachability data hasbeen subjected to a Tier 1 screening exercise using UK Environmental QualityStandards (EQS) for a saltwater environment (estuarine setting). A summary of thisscreening exercise is presented in Table No. 2. A more detailed quantitative riskassessment has not been carried out at this stage.

7.5.2 Of the metals, copper and selenium groundwater concentrations exceed EQSthresholds. Copper concentrations extracted through leachability testing on a samplerecovered from TP2 was well in excess of EQS, as were dissolved groundwaterconcentrations of selenium in BH-A. Otherwise, dissolved and leachableconcentrations were well below the relevant EQS threshold.

7.5.3 Of the organic contaminants, five exceedances were recorded in total. Allcontaminants exceeding EQS belonged to the PAH group (which have veryconservative thresholds). TPH was not detected in the dissolved phase. PAHexceedances were for naphthalene, anthracene and flouranthene.



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7.6 Previous Risk Assessment Comparisons

7.6.1 In summary the risk assessment carried out on the 2006 dataset generally agreeswith the findings reported back in 2001. In 2001, soil contaminant loadings were also

found to be generally low, with no evidence of significant site-wide contamination. Asfound during the 2006 study, the 2001 findings report isolated elevated concentrationsof heavy metals with a rare detections of elevated organic compounds in for exampleBH2 and BH3 (see Figure No. 3). Long term risk to human health is generallyconsidered to be low, provided certain straightforward mitigation measures are put inplace and provided any additional investigations below foundation slabs corroboratethis assessment.

7.6.2 Whilst human health risks may be concluded to be low, soil leachability characteristicsand dissolved concentrations of certain inorganic and organic contaminants detectedin the groundwater below the site suggest a precautionary approach to dealing withresidual contaminant sources may be required. Limited mitigation steps (primarilysource removal in certain areas) are warranted and should be encouraged in order toaddress a possible long term impact on surface water quality adjacent to the site.

7.7 Risk Assessment Conclusions

7.7.1 In the context of current UK Government guidance issued in connection with Part IIAof the Environmental Protection Act 1990, qualitative risks have been assessed usinga Source-Pathway-Receptor assessment approach. Such an approach recognisesthat risks from site based contaminants can only exist where all three components arepresent (constituting a complete pollutant linkage). The Conceptual Site Modelpresented as Figure Nos. 4-A and 4-B serves to illustrate existing and proposed sitespecific Source-Pathway-Receptor relationships and between the two demonstratesthe breaking of the identified linkages through the incorporation of a series of

pragmatic steps.

7.7.2 Although certain potentially significant contaminant sources were identified throughinvestigation in 2001, the levels of various PAH and metalloid compoundsencountered in shallow soils across the development footprint were generally belowthreshold guidance for human health risk. This is generally corroborated by thefindings of the 2006 investigations although further confirmatory investigation alongthe western site boundary is recommended. Contaminant levels exceeding thresholdguidance were typically at great depth (Arsenic 4.3 m depth - TP4), or straightforwardto mitigate by allowing for the provision of clean soil cover. Provision should be madefor a nominal 600 mm thickness of clean soil cover between any remaining MadeGround soils and future site users. This will mitigate any residual human health riskand tie in with ancillary flood defence measures being discussed by others.

7.7.3 Risks to groundwater below the site and in turn to human health are not consideredsignificant due to the environmental setting and the absence of any groundwaterabstractions in the vicinity of the site. The groundwater may however be viewed asviable pathway for the transmittal of residual contaminants in site soils to moreecologically sensitive surface water bodies adjacent to the site. For this reason, someform of remedial action (i.e. removal of the source term where practical), should beconsidered ahead of development.

7.7.4 Regarding soil gas and impacts to future development and future residents, furthergas monitoring of shallow soils is recommended to establish gas trends and to guidedesign. Based on existing data however (primarily the concentrations of Carbon



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Dioxide), it is considered likely that some form of gas protection measuresincorporated into foundation design will be necessary.



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SECTION 8

GEOTECHNICAL APPRAISAL


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SECTION 8 LAND AT NEWTOWN ROADGEOTECHNICAL APPRAISAL HIGHBRIDGE, SOMERSET

8 GEOTECHNICAL APPRAISAL

8.1 Ground Condition Summary

8.1.1 Two Cable Percussion boreholes were sunk and eight trial pits excavated across thesite during the recent 2006 investigation. The information gathered during the 2001investigation only included information on unit thickness and did not includegeotechnical data. The positions of all exploratory holes are shown on Figure No. 2.

8.1.2 The insitu. geotechnical testing data is presented on the borehole logs withinAppendix A. The laboratory geotechnical data is reported within Appendix C.

8.1.3 A summary of the strata encountered and the variation encountered is providedbelow:

Made Ground ranged in thickness between 0.3 m and 1.5 m. The Made Ground

comprised predominantly uncompacted, silty sandy gravel. The gravelcomponent of the Made Ground typically comprised fragments of brick and somewood. Sands were typically grey, mottled black and occasionally ashy.

Underlying the Made Ground, very soft to firm estuarine clay was encounteredattaining a thickness of up to 7 m. Standard Penetration Test (SPT-N) data forthis unit ranges between 1 and 14.

A Peat layer measuring 1.9 m in thickness was encountered in BH-A.

Below the estuarine clay unit a medium dense slightly gravely sand wasencountered, proven to a maximum depth of 13 m bgl. A thickness of at least3 m was proved in both BH-A and BH-B and Standard Penetration Test (SPT-N)data for this unit ranges between 16 and 32. The base of this unit was notproved but shown to extend at least 3.7 m below the estuarine clays.

8.1.4 Groundwater was encountered in both boreholes (water strikes coincident with theupper surface of the marine sands). Equilibrium rest water levels stabilised atapproximately 6.3 m bgl, slightly confined by the overlying estuarine clay.

8.2 Foundation Options

8.2.1 In general foundation solutions for the development should be relativelystraightforward to implement. Foundation options are likely to include the use ofeither ground bearing rafts or piles deriving their load capacity from the underlyingmarine sands. The ultimate selection of foundation solution will depend on the type ofconstruction envisaged and general development layout (both of which are yet to be

confirmed). Conventional two storey development may be able to proceed usingshallow foundation types such as ground bearing rafts founded on compacted fill (withperhaps some preparatory deeper ground improvement), whereas higher risedevelopment is likely to require a deeper, more robust solution such as a piledfoundation.

Made Ground / Structural Fill

8.2.2 The Made Ground is inherently heterogeneous and hence potentially highly variablein composition and behaviour during loading. Foundations should not be constructeddirectly within this horizon.



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SECTION 8 LAND AT NEWTOWN ROADGEOTECHNICAL APPRAISAL HIGHBRIDGE, SOMERSET

8.2.3 As part of the cover solution recommended for the site, provided the remaining MadeGround layer is proof rolled with soft spots removed and the cover soils used to raisesite levels are placed and compacted to an appropriate engineering specification,ground bearing rafts founded at the revised site levels should be achievable.

Estuarine Clay

8.2.4 The consistency of the estuarine clay below the Made Ground horizons ranges fromvery soft to firm and has a high organic content. This unit is likely to be variable inbehaviour and highly compressible under loading. Shallow foundations within thisunit are not recommended.

8.2.5 Additional compressibility data for the estuarine unit should be gathered prior todetailed design to better understand the frictional influence of this unit on any deeper,piled foundation solution considered necessary for high-rise development.Furthermore, a degree of preparatory ground improvement within the clay unit itself,using methods such as bottom-fed Vibro-Flotation stone columns, would also beadvised to add further support to any shallow ground bearing raft foundation typechosen for low rise development.

Marine Sand

8.2.6 A relatively competent gravely marine sand unit is present below the site. The depthat which this unit is available is likely to vary from east to west across the site (asindicated from BH-A and BH-B), but should be within 10 m of ground level across theentire proposed development platform. Based on the SPT-N data, this unit shouldprovide the necessary end bearing resistance required if a piled foundation solutionwere deemed the most appropriate for high rise development. It should be noted thatfactors such as negative skin friction should be built into any pile design to counter the

effects of any differential ground settlement expected within the overlying estuarinesequence.

8.2.7 Any floor slab construction required is likely to be over a site-wide covering of MadeGround soils. To allow for the dissipation of ground gases within this and below thisunit, it is therefore recommended that for any piled foundation solution all slabs besuspended above ground level to allow suitable venting. Various constructionmethods are available to achieve for example the use of pre-cast concrete beamswith block flooring systems designed with nominal ventilation spaces.

8.2.8 Alternatively, measures available to mitigate the potential for CO2 build up below aground bearing raft foundation include the use of either granular mattresses belowfoundation concrete or the use of an impermeable HDPE gas barrier such as

Visqueen

8.3 Foundation Concrete

8.3.1 Water soluble sulphate levels detected within the soils across this site are generallylow, ranging as they do between 0.043 and 0.049 g/l. pH values averageapproximately 7.5. The Aggressive Chemical Environment for Concrete (ACEC)classification for the site is AC-1, in accordance with BRE Special Digest 1, Concretein Aggressive Ground (2001).



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SECTION 9

CONCLUSIONS AND RECOMMENDATIONS


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SECTION 9 LAND AT NEWTOWN ROADCONCLUSIONS AND RECOMMENDATIONS HIGHBRIDGE, SOMERSET

9 CONCLUSIONS AND RECOMMENDATIONS

9.1 Current and Future Liabilities

9.1.1 In its current state, considering the proportion of hard cover across the site, thelinkage between existing soil contaminants and site occupiers is a tenuous one. Therisk to human health could therefore be considered acceptable for the time being. Forgroundwater, without the benefit of a more detailed quantitative risk assessment, therisks to neighbouring controlled waters receptors is more difficult to quantify. Theredoes appear to be some impact to groundwater quality below the site based on thegroundwater chemistry data available.

9.1.2 During any construction phase in the future, there are likely to be isolated hotspots ofground contamination encountered which should be addressed at this time. Re-development activity will inevitably disturb the prevailing equilibrium and could have aknock-on effect by impacting groundwater. In this regard, the most proactiveapproach to site preparation ahead of development would be to remove significantsoil contaminant sources thereby addressing any latent risk to groundwater/surfacewater quality.

9.2 Recommended Mitigation Measures - Future Use

9.2.1 The mitigation approach recommended in 2001 with regards to human health riskremains valid today based on the combined findings of two separate phases of work.In order to carry these measures forward effectively, the following general sequencefor ground preparation is recommended.

Existing building demolition followed by concrete segregation and crushing.

Targeted ground investigation below foundation slabs, particularly along thewestern site boundary of the site.

Targeted excavation and removal of contamination hotspots identified duringinvestigation (a source removal approach).

Provision of a nominal thickness of clean cover layer between any remainingMade Ground soils and future site users.

9.2.2 Further points of detail regarding the above are expanded upon below.

Allowance should be made for the targeted removal of residual hydrocarbons orashy soils as development proceeds. As the development site opens up, or

becomes more accessible to ground working, delineation of the contaminantsource areas identified will become easier, allowing selective excavation andremoval to take place where necessary. As a guide, efforts will need to befocussed initially toward the western site boundary, around BH1, 2, 3 and TP7and below current foundations slabs in this area, as well as around TP1, 2 andBH4 toward the eastern site limit. Likely costs to address this issue arediscussed in Section 9.4.

Clean cover soils should typically be no less than 600 mm in thickness. Any sitewon excavated concrete generated during the construction phase may becrushed and contribute towards raising site levels and act as a capillary breaklayer at the base of any imported materials (provided they are suitably clean).An imported source of clean top soil may also be required to add cover to areaswhere any dedicated planting is envisaged.



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In areas where site sensitivity is deemed to be relatively low (e.g. below accessroads or areas of proposed hard-standing), cover thickness requirements for thepurpose of providing a barrier to contamination may be reduced to suit generalsite level design.

In areas where soils are to be used for the growing of potentially sensitiveplanting Made Ground would need to be excavated to full depth and replacedwith clean imported soil, suitable as a growing medium. Alternatively if feasible,raised planting beds should be considered.

Around the fringes of the overall development, where site levels need to featherin with the surroundings (e.g. to tie in with adjoining residential boundaryelevations), localised over excavation of Made Ground soil may be required,depending on ultimate land use, to maintain a 600 mm clean cover barrier.

In the vicinity of the access road to the development, hard tarmac/concrete coverwill provide a suitable barrier to shallow sub-surface soil contaminationencountered for example in TP6 and BH5. Cover requirements in this area could

be relaxed to allow levels to tie in with existing access gradients alongNewtown Road.

Any soil re-use / import strategy will need to be approved with the LocalAuthority. It is likely that some form of independent supervision and validationwill be required to show that soil import or placement is handled correctly. Oncethe soils are placed, it will be important to be able to demonstrate that soilcontaminant concentrations in each area remain consistent and suitable for use.In this regard, the Local Authority will require confirmation that the thickness andquality of topsoil/subsoil in gardens and landscaped areas is in accordance withapproved criteria.

9.3 Recommended Mitigation Measures - Construction Period

9.3.1 During site re-development, in particular during earthworks, Health and Safetyprotocols with respect to hygiene will be required to protect the health of workers andgeneral public. The data included in this report should therefore be incorporated intodesigner risk assessments (as required by CDM Regulations). The protocols adoptedwould not be expected to compromise the development, and would be no more thanusually required for brownfield development (e.g. dust suppression, good hygiene andwaste management practices etc).

9.4 Mitigation Costs

Off-Site Disposal

9.4.1 The 2001 study estimated that it was likely that approximately 1000 m3

of soils wouldrequire treatment or off-site disposal. In light of the 2006 investigations, we considerthis volume assessment to be reasonable considering the continued uncertainty oversoil quality below existing structures and between existing sampling points.

9.4.2 Key legislation changes over the past five years urge a sustainable approach to theredevelopment of brownfield sites by discouraging indiscriminate off-site soil disposalto landfill (dig and dump). The implementation of the Landfill Directive in 2004/05, therequirement for Waste Acceptance Criteria (WAC) testing to be undertaken on wastesources prior to landfill disposal and the greater distances materials have to betransported today, has had significant cost implications for off-site soil disposal fromdevelopment sites.



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9.4.3 Depending on waste classification (Non-Hazardous or Hazardous), off-site disposalcosts for 1000 m3 of material in 2006, could feasibly range between 45,000 and100,000 (based on between 45 per m

3and 100 per m

3). This is significantly

greater than identified in 2001. Clearly the emphasis needs to be on reaching a

mutually agreeable approach to site development between the regulatory bodies andthe ultimate site developer, minimising the need for off-site disposal. WasteAcceptable Criteria (WAC) testing should be undertaken as a matter of course toprovide better cost certainty.

Soil Importation

9.4.4 Based on available site details, approximately 15% of the materials required to raisethe development platform should be available on site (site won crushed concrete).The remainder (a conservative estimate of 13,000 m

3), will need to be sourced

through an off-site supplier. Approximate unit costs to import, place and compact asuitable granular fill to an engineering specification would be in the region of 18 -24/m

3.

9.5 Geotechnical Issues

9.5.1 In general, foundation solutions for the development should be relativelystraightforward to implement. Foundation options are likely to include the use ofeither ground bearing rafts or piles deriving their load bearing from the underlyingmarine sands. The ultimate selection of foundation solution will depend on the type ofconstruction envisaged and general development layout (both of which are yet to beconfirmed). Conventional two storey development may be able to proceed usingshallow foundation types such as ground bearing rafts founded on compacted fill (withperhaps some preparatory ground improvement), whereas higher rise development islikely to require a deeper, more robust solution such as a piled foundation.



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TABLES


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FIGURES


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APPENDICES


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APPENDIX A

EXPLORATORY HOLE LOGS


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APPENDIX B

CHEMICAL TEST RESULTS


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APPENDIX C

GEOTECHNICAL TEST RESULTS


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APPENDIX D

CLEA UK DATASHEET SUMMARIES

Documents

010-158-42167 Ground Investigation Report